What are valve positioners?
Positioners are used to allow the final control element to fully perform its function in the control loop. They put the valve in the correct position by increasing or decreasing the air load pressure on the actuator.
Functions of a valve positioner:
As they are installed in a direct control valve, they can be influenced by vibration in the piping, temperature, environmental gases, etc. Thus they must have a solid construction. These are the cases where we uses valve positioners:
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The differential pressure of the valve is great and the effects of change in fluid pressure are strong.
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Friction with the gland packing is great and hysteresis and linearity become bad.
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The controller signal and actuator signal differ.
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The distance between the controller and control valve is large and response is slow (for pneumatic signals).
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The intake diameter of the control valve exceeds 100 mm, and the response is slow due to heavy load.
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Two or more control valve operate in a split range in reponse to one signal
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The operation signal is amplified because a large operating power is required
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The pressure of the fluid has a strong effect, as with butterfly valves.
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it is necessary to improve the inherent characteristics of the valve to obtain the required characteristics.